1. Field of the Invention
The present invention relates to a stacked piezoelectric element for use as a stacked electro-mechanical energy conversion element, which has a configuration particularly suitable for miniaturization.
2. Related Background Art
Conventionally, a piezoelectric material having an electro-mechanical energy conversion function has been used as various types of piezoelectric elements and piezoelectric devices. Particularly, regarding these piezoelectric elements and piezoelectric devices, a recent tendency has been to use, for example, those having a structure constituted by laminating a large number of single plate-like sheets formed by ceramics.
This is because, in contrast to a single plate-like piezoelectric element, through lamination, a large deformation distortion as well as a large power can be obtained with a low applied voltage. In addition, a sheet formation method and a manufacturing method of lamination are so generalized that a thickness of one layer to be laminated can be made thinner, thereby making it easy to obtain the piezoelectric element of a small size and a high performance.
For example, a stacked piezoelectric element for use in a vibration wave motor as a vibration wave drive device is proposed in U.S. Pat. No. 6,046,526 or U.S. Pat. No. 5,770,916. In addition, a large number of stacked piezoelectric elements for use in a vibration gyro and a piezoelectric transformer are also proposed.
With respect to the stacked electro-mechanical elements used for such a variety of applications, those having an electrode area formed in a material having the electro-mechanical energy conversion function, for example, those having a structure constituted by superposing the electrode in a plurality of layers which are formed of an electrode material and disposed on the surface of the piezoelectric ceramics (hereinafter referred to as the internal electrode) are used.
In general, as an inter layer wiring line for connecting one internal electrode of the layers to another having a different laminated state, an electro-conductive film (hereinafter referred to as the external electro-conductive film) used to form a connection is disposed on an outer periphery or an inner periphery of the stacked piezoelectric element, or a hole is disposed inside the layer of a piezoelectric layer in which the electrode material is embedded, thereby defining a through-hole (a via hole).
FIGS. 5A and 5B and FIGS. 6A and 6B show the conventional stacked piezoelectric element described in U.S. Pat. No. 5,770,916. In FIGS. 5A and 5B, a disc-shaped stacked piezoelectric element 110, in the center portion of which a penetration hole is formed, is constituted by the uppermost piezoelectric sheet 102 and a plurality of piezoelectric sheets 112. In one surface (hereinafter referred to as a first surface) of each of a plurality of piezoelectric layers (sheets) 112 constituting the stacked piezoelectric element, there are formed an internal electrode 113. This internal electrode 113 is constituted by an internal electrode pattern having four divided structures which are mutually non-conductive, and a plurality of piezoelectric sheets 112 are laminated in such a manner that the phases of the divided patterns of the internal electrodes 113 correspond to each other. In the outer peripheral portion of each divided pattern of the internal electrode 113, there is formed a connecting pattern 103a and this connecting pattern 103a reaches the outer peripheral end of the piezoelectric sheet 112. At this time, the connecting pattern 103a is located at the same phase in every other piezoelectric sheet 112.
By an external electro-conductive film 114 which is disposed on the outer peripheral surface of the laminated piezoelectric sheet, the connecting pattern 103a of every other piezoelectric sheet 112 is mutually communicated. In the uppermost piezoelectric sheet 102, a surface electrode (a terminal pattern) 115 is formed on its outer peripheral end and communicated with the external electro-conductive film 114.
On the other hand, the stacked piezoelectric elements of FIGS. 6A and 6B obtain communications among laminated piezoelectric layers (sheets) utilizing through-holes. On the surface of each piezoelectric sheet 112 constituting the stacked piezoelectric element 111, there is disposed the internal electrode 113 constituted by the four divided structures, and in the vicinity of the inner peripheral side of each piezoelectric sheet, there are formed through-holes 116 as illustrated in the drawing by black dots. Of the through-holes 116, there are those having communications with the internal electrodes 113 and those having no communications, and in this case, they are mutually located at positions shifted 90 degree in phase. The piezoelectric sheets 112 are laminated in such a manner that the internal electrodes 113 have 90 degree phase difference for every other sheet. In this way, the through-holes 116 located in the same phase are in a state of being alternatively superposed with those having communications with the internal electrodes 113 of the piezoelectric sheets 112 and those not having communications. In every odd number of the piezoelectric sheets 112, internal electrodes which communicate via through-holes 116 are aligned with one another in the axial direction of the stacked piezoelectric element, and in every even number of the piezoelectric sheets 112, internal electrodes which communicate with the through-holes 116 are aligned with one another in the axial direction of the stacked piezoelectric element. In this way, through-holes 116 which communicate with the internal electrodes 113 disposed in every odd number of the piezoelectric sheets 112 are aligned and connected with the non-communicating through-holes 116 of every even number of sheets. Similarly, through-holes 116 which do not communicate with odd numbered sheets are aligned with through-holes 116 which do communicate with even numbered sheets. Each through-hole 116 exposes the end portion of the through hole 116 at the uppermost piezoelectric sheets 102 of the stacked piezoelectric element 111 to form a surface electrode 117.
The stacked piezoelectric elements of FIGS. 5A and 5B and FIGS. 6A and 6B thus constituted are subjected to a polarization process and exhibit piezoelectric efficiencies.
FIG. 7 is a view incorporating the stacked piezoelectric element 110 or 111 into a vibration body 120 constituting a vibration wave motor as a bar-shaped vibration wave driving device. The stacked piezoelectric element 110 (111) is sandwiched between metal parts 121, 122 which are elastic members of the vibration body 120 by a bolt 123 through a wiring substrate 118 to be connected with an external power source. The wiring substrate 118 is electrically connected to each surface electrode 115 (117) of the stacked piezoelectric element 110 (111) and generates a driving vibration attributable to the synthesis of two bending vibrations orthogonal to the vibration body 120. By this driving vibration, a rotor 132 which is brought into press contact with an elastic member 121 by a spring 130 and a spring supporting body 131 is frictionally driven, so that a driving power is output by a gear 133 working as an output member integrally rotating with the rotor 132.
It is to be noted that the stacked piezoelectric element 111 which uses the whole of through-holes for connecting the internal electrodes of FIGS. 6A and 6B has already been mass-produced and come in practice as a motor to be used for the auto-focus of camera lenses incorporated into the vibration wave motor as shown in FIG. 7.
The stacked piezoelectric elements of the external electrode connection system as shown in FIGS. 5A and 5B and the through-hole connection system as shown in FIGS. 6A and 6B are constituted by the electrode portions (patterns) in which the internal electrodes 113 of each layer inside the elements is divided into four parts and, as illustrated in the drawings, are polarized (indicated by +and xe2x88x92) in such a manner that two electrode portions spaced at 180 degree are mutually different in a polarization direction. These are taken as one pair of (A+, Axe2x88x92) and (B+, Bxe2x88x92) which are taken as a phase A and a phase B, respectively and, in an opposite layer (hereinafter referred to as a second driving layer) to this one layer (hereinafter referred to as a first driving layer), similarly two electrode portions which are equivalent to ground and spaced at 180 degree are taken as one pair which are taken as a phase AG and a phase BG, respectively. In the same phase of each layer, each electrode portion of the same polarization direction (one another of electrode patterns of the same phase of the above described each first driving layer and one another of electrode patterns of the same phase of the above described each second driving layer) is electrically connected by the through-hole as an inter-layer electrode or the external electro-conductive film. To the phase A and the phase B which is 90 degree different from the phase A in the spatial phase position, a high frequency wave voltage, which is approximately in accord with natural vibration of the vibration body, is applied, and, as the ground, the phase AG and the phase BG opposing to the phase A and the phase B generate two bending vibrations orthogonal to the vibration body.
However, in order to develop a future small-sized vibration wave motor, when an attempt is made to miniaturize the stacked piezoelectric element from the conventional diameter of 10 mm to a diameter of 6 mm, a ratio of the area of the insulating portion (non-electrode forming portion) of the through-hole and its periphery and the whole area of the internal electrode becomes larger than the conventional ratio. As a consequence, an effective area of the piezoelectric activation portion for driving the vibration wave motor is not made larger and a driving efficiency of the motor is not expected to increase appreciably.
Again, when an attempt is made to manufacture the conventional external electro-conductive film as an inter-layer electrode using a low-cost screen printing method, as compared with the case of using through-holes, unevenness tends to develop in the height from the piezoelectric layer surface of the surface electrode 115.
Consequently, in miniaturizing the stacked piezoelectric element, it is considered that there is much room left for improvement in order to achieve both the feature of securing an effective area of the piezoelectric activation portion and maintaining processing accuracy of the surface electrode.
One aspect of the present invention is to make a stacked electro-mechanical energy conversion element having a piezoelectric layer so as to carry the piezoelectric layer having electro-conductive film for allowing the external electro-conductive film and a through-hole to communicate. In this way, the inter-layer piezoelectric layer is constituted by having the external electro-conductive film, so that the effective area of the piezoelectric activation portion is made larger, and the electrode layer connected with the external power source is constituted by having the through-hole, so that contact accuracy with the external power source can be enhanced.